Rare earth (RE) elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and others) are produced in nuclear fuel by uranium fission and by the decay of other fission products. Many of the rare earths have a large neutron cross section so that they make a large contribution (approximately 50%) to the neutron burden of spent nuclear fuel. When the neutron burden becomes too high, nuclear fuel must be removed from the reactor for disposal. An effective method for removing the RE would allow spent fuel to be reused, and thus provide more energy per kilogram of starting material.
Several dry processing techniques, based on air oxidation and thermal treatment of spent nuclear fuel, are available; none of these effect the removal of any significant amounts of rare earths.
Two of the known dry processing techniques are the AIROX and OREOX processes. In the AIROX process, fuel decladding can be accomplished oxidatively or by conventional mechanical means. In oxidative decladding, the fuel pin is punctured and then heated in air (400.degree. to 600.degree. C.) so that the oxidation of UO.sub.2 to U.sub.3 O.sub.8 causes the cladding to rupture. The resulting U.sub.3 O.sub.8 powder can then be easily separated from the cladding. The U.sub.3 O.sub.8 is then reduced in hydrogen at 600.degree. to 1100.degree. C. to regenerate UO.sub.2. The oxidation/reduction steps are performed at high enough temperatures to cause the release of volatile fission products. By using oxidation/reduction cycling, the AIROX process can achieve almost complete removal of Xe, Kr, Cs and I.
The OREOX process is an improvement on the AIROX process. Oxidation is performed at a higher temperature (1200.degree. C.) than in the AIROX process resulting in a more effective removal of the volatile fission products.
Wet reprocessing techniques (based on fuel dissolution and subsequent chemical separation) can be used to remove rare earth elements from spent fuel but are not commercially viable because of the large volumes of liquid waste generated and also because of the need to maintain strict plutonium diversion safeguards. For example, Canadian Patent 589,122 discloses a method of reprocessing irradiated nuclear reactor fuel. The patent discloses removal of 99% of some rare earth elements and comprises contacting the uranium in a molten state with a refractory oxide under non-oxidizing conditions and separating the decontaminated uranium from the fission products-containing oxides.
U.S. Pat. No. 2,822,260 discloses a process for the separation of rare earths and other fission product metal values from neutron bombarded uranium. The patent discloses melting uranium with a metal oxide at a temperature from about 1150.degree. to 1400.degree. C. in an inert atmosphere to produce a scale of uranium dioxide on the uranium which is strongly concentrated with most of the fission products.
The present invention relates to a dry processing technique which enables the removal of a significant portion of rare earth elements from irradiated uranium dioxide fuels.